Extra-vehicular activity
From Wikipedia, the free encyclopedia
Extra-vehicular activity (EVA) is work done by an astronaut away from the Earth and outside of a spacecraft. The term most commonly applies to an EVA made outside a craft orbiting Earth (a spacewalk) but also applies to an EVA made on the surface of the Moon (a moonwalk). In the later lunar landing missions the command module pilot did an EVA to retrieve film canisters on the return trip.
Due to the different designs of the early spacecraft, the American and Soviet space programs also define an EVA differently. Russians define an EVA as occurring when a cosmonaut is in a vacuum. An American astronaut, in contrast, is not considered to have made an EVA until at least his head is outside the spacecraft. The term stand-up EVA (SEVA) is used for being partly outside.
EVAs may be either tethered (the astronaut is connected to the spacecraft, oxygen can be supplied through a tube, no propulsion is needed to return to the spacecraft) or untethered. When the tether performs life support functions such as providing oxygen, it is called an umbilical. For untethered EVAs during space flight, capability of returning to the spacecraft is essential; see Manned Maneuvering Unit (MMU).
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[edit] EVA milestones
- The first EVA was carried out by Soviet cosmonaut Aleksei Leonov on March 18, 1965 from the Voskhod 2 spacecraft.
- The first EVA by an American astronaut was made on June 3, 1965 by Edward White during the Gemini 4 mission.
- The first EVA that was a moonwalk rather than a spacewalk was made by American astronaut Neil Armstrong on July 20, 1969 when the Apollo 11 Lunar Module Eagle landed on the Moon. He was joined by crewmate Buzz Aldrin, and their EVA lasted 2 hours and 32 minutes.
- The first untethered spacewalk was by American astronaut Bruce McCandless II on February 7, 1984, during Challenger mission STS-41-B. He was subsequently joined by astronaut Robert L. Stewart during the 5 hour 55 minute spacewalk.
- The first woman to perform an EVA was Cosmonaut Svetlana Savitskaya on July 25, 1984 while aboard the Salyut 7 space station.
- The first American woman to make an EVA was Kathryn D. Sullivan, who stepped into space on October 11, 1984 during Space Shuttle Challenger mission STS-41-G.
- The first EVA by a non-Soviet, non-American astronaut was made on December 9, 1988 by Jean-Loup Chrétien of France during a three-week stay on the Mir space station.
- The first EVA to perform an in-flight repair of the Space Shuttle was by American astronaut Steve Robinson on August 3, 2005, during "Return to Flight" mission STS-114. Robinson was sent to remove two protruding gap fillers on the Space Shuttle Discovery's heat shield, after engineers determined they might cause damage to the shuttle upon re-entry. Robinson successfully removed the loose material while the Discovery was docked to the International Space Station.
- The longest EVA was 8 hours and 56 minutes, performed by Susan J. Helms on March 11, 2001.
- Anatoly Solovyev, with 16 spacewalks, totaling 77 hours, 41 minutes EVA, holds the records for most spacewalks and greatest duration on EVA.
[edit] EVA hazards
An EVA is dangerous for a number of different reasons. The primary one is collision with space debris. Orbital velocity at 300 km above the Earth (typical for a Space Shuttle mission) is 7.7 km/s. This is 10 times the speed of a bullet, so the kinetic energy of a small particle with a mass 1/100th that of a bullet (e.g. a fleck of paint or a grain of sand) is equal to that of a bullet. Every space mission creates more orbiting debris, so this problem will continue to worsen (see also Kessler Syndrome).
Another reason for danger is that external environments in space are harder to simulate before the mission, though approximate simulations can be achieved at facilities like NASA's Neutral Buoyancy Laboratory. Space walks are avoided for routine tasks because of their danger. As a result the EVAs are often planned late in the project development when problems are discovered, or sometimes even during an operational mission. The exceptional danger involved in EVAs inevitably leads to emotional pressures on astronauts.
Other possible problems include a space walker becoming separated from their craft or suffering a spacesuit puncture which would depressurize the suit, causing anoxia and rapid death if the space walker is not brought into a pressurized spacecraft quickly.
One astronaut has suffered a spacesuit puncture. During STS-37, a small rod punctured the glove of one of the astronauts (the name is undisclosed, but it was either Jerry L. Ross or Jay Apt). However, the puncturing object, which stabbed the astronaut's hand as well, held in place, resulting in no detectable depressurization. In fact, the puncture was not noticed until after the space walkers were safely back inside Atlantis. [1]
Aleksei Leonov's EVA did not pass smoothly, although this was not reported at the time. Reportedly his spacesuit was overinflated due to vacuum conditions and so had to be vented, in order to return through the airlock. Because Leonov was breathing an "Earth-normal" mixture, the nitrogen in his blood briefly bubbled, resulting in a case of "the bends", normally suffered by deep-sea divers when they ascend to the surface too fast.
For EVAs from ISS NASA now routinely employs a camp out procedure to reduce the risk of decompression sickness. This was first tested by the Expedition 12 crew.[1]
As of 2007, no catastrophic incident has ever occurred during an extra-vehicular activity, and no astronaut or cosmonaut has ever died during one. However, given the considerable hazards inherent in EVAs, and the resultant risk to astronauts, some scientists are working to develop tele-operated robots for outside construction work, to potentially eliminate or reduce the need for human EVAs.
[edit] See also
[edit] References
- ^ International Space Station Status Report #06-7 http://spaceflight.nasa.gov/spacenews/reports/issreports/2006/iss06-7.html
[edit] External links
- NASA JSC Oral History Project Walking to Olympus: An EVA Chronology PDF document.
- NASDA Online Space Notes
- Apollo Extravehicular mobility unit. Volume 1: System description - 1971 (PDF document)
- Apollo Extravehicular mobility unit. Volume 2: Operational procedures - 1971 (PDF document)
- Skylab Extravehicular Activity Development Report - 1974 (PDF document)
- Analysis of the Space Shuttle Extravehicular Mobility Unit - 1986 (PDF document)
- NASA Space Shuttle EVA tools and equipment reference book - 1993 (PDF document)
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